English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Previous investigations have indicated that the Caribbean gorgonian coral Plexaura homomalla derives a chemical defense from prostaglandins present at high concentrations w~th in the coral's soft tissues. These conclusions were based on laboratory and field experiments with the hydroxy acids of prostaglandin A2 (PGA2), and not the acetoxy methyl esters of PGA2 that are present in the living gorgonian. In the present study, the consumption of food strips containing the acetoxy methyl ester of 15(R) PGA, from P. homomalla by a natural assemblage of Caribbean coral reef fishes did not differ from consumption of control stnps. In support of previous findings, treatment with the acetoxy acid, hydroxy methyl ester and hydroxy acid of 15(R) PGA2 inhibited consumption of food strips by reef fishes. Although the acetoxy methyl esters of PGA2 undergo hydrolysis when the soft tissues of P. homomalla are damaged, the process appears to be too slow to provide the gorgonian with an effective inducible defense mechanism. Some species of Caribbean gorgonian corals of the genus Plexaura (in particular, P. homomalla) contain high concentrations of prostaglandins within their soft tissues; estimates range from 1 to 8 % of wet tissue weight (Schneider et al. 1977, Domingez et al. 1980) and 1 to 2 O/O of dry tissue weight (Weinheimer & Spraggins 1969). Non-esterified prostaylandins mediate a variety of biological responses, particularly in mammals, and are generally found in only trace concentrations in animal tissues (Nomura & Ogata 1976). The biological function of such high concentrations of these compounds in the tissues of P. homomalla is unknown, particularly because CO-occurring species of gorgonians do not contain prostaglandins in appreciable amounts. Equally noteworthy, fully-esterified prostaglandin A2 (PGA2; Fig. ID), the most abundant Present address: Friday Harbor Laboratones, University of Washington, 620 Unlverslty Road, Friday Harbor, Washington 98250. USA :C Inter-Research/Printed in F. R. Germany COOR , 1 OR: A. FIYDROXY ACID R,=H. R,=H B. H Y D R O X Y CIETHYL ESTEK: R,=cH,. R ? = ~ I C. ACETOXY ACID. R,=H. R,=COCH, D. ACETOXY METHYL ESTER: R,=CH,. R2=COCH, Fig. 1. Structure of 15(R) prostaglandn A2 (PGA2) from the gorgonian coral, Plexaura homornalla (A). Structures of the 3 denvatives assayed In this study are also indicated (B, C , D) prostaglandin in the tissues of P. homomalla, occurs predominantly in the 15(R) isomer rather than the 15(S) form found in all other animal tissues. The 15(R) isomer is much less active than the 15(S) in representative pharmacological assays (Spraggins 1972). Gerhart (1984) tested the hypothesis that the prostaglandins in Plexaura homomalla function as a chemical defense against predatory reef fishes. He found that both the 15(S) and 15(R) isomers of unesterified PGA2 induced regurgitation in the yellowhead wrasse Halichoeres garnoti in feeding experiments conducted on coral reefs off Curasao, and obtained similar results in laboratory assays with the killifish Fundulus heteroclitis (Gerhart 1984). As Gerhart pointed out, however, the compounds used in his assays were the hydroxy acids of 15(S) and 15(R) PGA2 (Fig. 1A) (Gerhart 1984, p. 183), while the acetoxy methyl esters (Fig. ID) of these compounds are the sole derivatives stored in the living tissues of the gorgonian (Schneider et al. 1977). Gerhart jus96 Mar. Ecol. Prog. Ser. 52: 95-98. 1989 tified his use of the hydroxy acids because they are the ultimate products of enzymatic hydrolysis of the acetoxy methyl esters after the coral tissue is damaged; however, completion of this process is reported to take -24 h (Schneider et al. 1977). Gerhart (1986) also tested the hypotheses that prostaglandins function as allelopathic agents or as inhibitors of blofouling of Plexaura homomalla. He found little or no PGA2 in the seawater surrounding colonies of P. homomalla, and neither isomer of PGA2 inhibited the fouling of gorgonian axial skeletons. It is unclear whether the hydroxy acids or acetoxy methyl esters of PGAl were employed in the fouling assays. In a survey of the chemical defenses of Caribbean gorgonian corals, we found that extracts of most of the gorgonian species of the genus Plexaura were palatable to the bluehead wrasse Thalassoma bifasciatum in laboratory assays (Pawlik et al. 1987). One sample, Plexaura sp. voucher No. 29, yielded an extract containing esterified prostaglandins. This extract was unpalatable in preliminary assays, but did not significantly deter fish feeding in more rigorous assays conducted at diminishing concentrations (Pawlik et al. 1987, Table 11). The purpose of the present study was to ascertain whether naturally-occurring prostaglandins contained in the tissues of P, homomalla (i.e., the acetoxy methyl esters) provide an effective chemical defense against a natural assemblage of predatory reef fishes in field assays. Materials and methods. Prostaglandins employed in the assays were purified from a frozen, crude dichloromethane extract of Plexaura homomalla collected in August 1983 a t Acklins Island, Bahamas. The gorgonians had been air-dried prior to extraction. Extracts contained a mixture of the acetoxy methyl ester, hydroxy methyl ester and hydroxy acid (approximate ratio of 2 : 1: 1, respectively) of 15(R) PGA2 (Fig. l) , as identified by NMR spectroscopy The 15(S) isomer of PGA2 was not detected. The prostaglandins in the extract were purified by flash chromatography on silica gel employing a 0 to 100 O/O ethyl acetate in iso-octane gradient, followed by silica gel HPLC on a preparative column with 2: 1 hexanes:ethyl acetate as the eluant. A portion of the purified hydroxy acid was acetylated with acetic anhydride in pyridine to yield the acetoxy acid (Fig. 1C). Purified samples were kept frozen for the 1 to 2 wk prior to use in field assays. Extracted, esterified prostaglandins were stable to further hydrolysis. Field assays were conducted near Looe Key Reef, south of Big Pine Key. Florida, USA, in September 1987. Prostaglandins were incorporated into a matrix of 4 % carageenan in the form of 1.0 X 0.5 X 5.0 cm strips (Harvell 81 Fenical 1988). Each strip enclosed a length of cotton string that protruded from one end of the strip. Strips were made by adding water to 2.5 g carageenan (Gelcann, FF961L; FMC Corp., Philadelphia) and 0.6 g freeze-dried brine shrimp (Artemia sp.) to a total volume of 60 m1 and heating the mixture to boiling. To this mixture, 1.5 g of prostaglandin were added in a minimal amount of ethanol. The mixture was stirred and then poured into plastic molds crossed by lengths of cotton string. After the matrix cooled and gelled, the strips were sliced to size with a scalpel and removed from the mold. Control strips contained an equivalent volume of ethanol, but no prostaglandin. The assay concentration of 1.5 g prostaglandin in a 60 m1 carageenan mixture (-2.5 O/O by volume) is a high value relative to the levels of prostaglandins found in Plexaura homomalla. We calculated -0.86 g of dry P homomalla tissue per m1 of wet tissue volume (excluding gorgonin axial skeleton). Assuming that 2 O/O of the dry weight of the gorgonin is composed of prostaglandins (Weinheimer & Spraggins 1969), then 60 m1 of this tissue would contain -1.0 g of prostaglandins (-1.7 % by volume). Prostaglandin concentrations of -2.5 % by volume (-3 O/O dry weight) would be at the upper limit of their natural concentration in P. homomalla. Therefore, any conclusions drawn from assays conducted at this concentration on the ineffectiveness of the compounds to deter predation would be conservative. One treatment and one control strip each were affixed to a 50 cm length of 3-strand nylon rope (at a distance of -4 and 14 cm from one end) by tying the cotton string protruding from each strip through the strands of the rope. Twenty such ropes were deployed on the reef for each experiment, with the end of each rope opposite the food strips unwound and clamped onto a piece of coral or rock at 10 to 15 m water depth. After 1 to 4 h, the ropes were retrieved from the reef and the amount of each strip eaten was recorded as a percentage decrease in the strip eaten was recorded as a percentage decrease in the strip length (to the nearest 5 %). Strip length was adjusted for bites taken out of the edges of the strip (see Hay 1984, for a detailed discussion of these assay methods). The Wilcoxon paired-sample test (one-tailed, Zar 1984) was employed to analyze the assay results after first excluding pairs for which both control and treatment slices had been either completely eaten, or not eaten at all. Results and discussion. In support of Gerhart's (1984) results, the hydroxy acid of 15(R) PGA2 from Plexaura homomalla (Fig. 1A) inhibited the feeding of a natural assemblage of reef fishes (Fig. 2A). The principal reef fishes observed feeding on assay strips were wrasses, primarily Thalassoma bifasaatum, and 2 or 3 species of Halichoeres. Damselfishes, Abudefduf sp. in particular, were occasionally observed biting at the strips. Both partially-esterified prostaglandins, the hydroxy methyl ester (Fig. 1B) and the acetoxy acid (Fig. l C ) , Pawlik & Fenical: Ichthyodeterrence of coral prostaglandins 97

A re-evaluation of the ichthyodeterrent role of prostaglandins in the Caribbean gorgonian coral Plexaura homomalla